In order to approximate the appearance of continuous-tone (photographic) images via ink-on-paper printing, the commercial printing industry relies on a process known as halftone printing. In halftone printing, color density gradations are produced by printing patterns of dots or areas of varying sizes, but of the same color density, instead of varying the color density continuously as is done in photographic printing.
There is an important commercial need to obtain a color proof image before a printing press run is made. It is desired that the color proof will accurately represent at least the details and color tone scale of the prints obtained on the printing press. In many cases, it is also desirable that the color proof accurately represent the image quality and halftone pattern of the prints obtained on the printing press. In the sequence of operations necessary to produce an ink-printed, full-color picture, a proof is also required to check the accuracy of the color separation data from which the final three or more printing plates or cylinders are made. Traditionally, such color separation proofs have involved silver halide photographic, high-contrast lithographic systems or non-silver halide light-sensitive systems which require many exposure and processing steps before a final, full-color picture is assembled.
There is also an important commercial need to obtain a halftone color proof directly from digital color separation data.
In order to produce a halftone pattern, the process used must be capable of high resolution, i.e., at least 60 pixels/mm. To achieve that resolution, the process must be, locally, adiabatic, with no or little thermal flow to surrounding areas. Localized temperatures in these processes will range from about 500.degree. C. to over 1500.degree. C. using a laser.
Well known thermal transfer processes which employ an array of resistive heating elements as the imaging source are, locally, isothermal, which involves thermal flow to surrounding areas. Being isothermal, these processes lack the resolution for producing an accurate halftone color proof.
In U.S. Pat. No. 5,126,760, a process is described for producing a direct digital, halftone color proof of an original image on a dye-receiving element. The proof can then be used to represent a printed color image obtained from a printing press. The process described therein comprises:
a) generating a set of electrical signals which is representative of the shape and color scale of an original image; PA1 b) contacting a dye-donor element comprising a support having thereon a dye layer and an infrared-absorbing material with a first dye-receiving element comprising a support having thereon a polymeric, dye image-receiving layer; PA1 c) using the signals to imagewise-heat by means of a diode laser the dye-donor element, thereby transferring a dye image to the first dye-receiving element; and PA1 d) retransferring the dye image to a second dye image-receiving element which has the same substrate as the printed color image.
In the above process, multiple dye-donors are used to obtain a range of colors in the proof. For example, for a full-color proof, four colors: cyan, magenta, yellow and black are normally used.
By using the above process, the image dye is transferred by heating the dye-donor containing the infrared-absorbing material with the diode laser to volatilize the dye, the diode laser beam being modulated by the set of signals which is representative of the shape and color of the original image, so that the dye is heated to cause volatilization only in those areas in which its presence is required on the dye-receiving layer to reconstruct the original image.
In color proofing in the printing industry, it is important to be able to match the proofing ink references provided by the International Prepress Proofing Association. These ink references are density patches made with standard 4-color process inks and are known as SWOP.RTM. (Specifications Web Offset Publications) color aims. For additional information on color measurement of inks for web offset proofing, see "Advances in Printing Science and Technology", Proceedings of the 19th International Conference of Printing Research Institutes, Eisenstadt, Austria, June 1987, J. T. Ling and R. Warner, p.55.
Although a wide gamut of printing ink colors can be matched by just a few dye-donor elements, there are certain types of inks used in the printing industry which cannot be matched by any combination of dyes. Notable among these types of pigments are the metallics, white and opaque spot colorants.
In U.S. Pat. No. 5,278,023, there is disclosed a propellant-containing laser thermal transfer donor element which is used to mass transfer pigments or dyes to a receiver element. However, there is no indication that only a portion of dye from a dye-donor element could be transferred in combination with a mass transfer of a pigment layer.
In U.S. Pat. No. 5,464,723, a process is described using thermal mass transfer of a white or metallic donor layer followed by a thermally transferred dye image. However, there is a problem with this process as it employs an array of resistive heating elements as the imaging source which are isothermal, involving thermal flow to surrounding areas. Therefore, this processes lacks the resolution for producing an accurate halftone color proof.
It is an object of this invention to provide a halftone color proof that better represents the color gamut of a color image obtained from a printing press.